The present invention relates to a thermal type flow sensor that measures a flow rate of a fluid such as air flow by using a heat resistive element (temperature sensitive resistive element) having a temperature dependency.
The thermal type flow sensor controls a current flowing through a heat resistive element so as to maintain a temperature difference between the heat resistive element whose heat is taken away with the fluid to be measured and a temperature compensation resistor a constant temperature difference. In order to carry out the current control, the thermal type flow sensor includes a bridge circuit with the heat resistive element and the temperature compensation resistive element and a driver circuit which controls the current flowing through the bridge circuit so that a potential difference between two middle points of the bridge circuit becomes zero. The driver circuit comprises a differential amplifier circuit and a transistor.
The thermal type flow sensor is suitable for a sensor that measures a intake air flow rate of, for example, a vehicle engine because the thermal type flow sensor is capable of directly measuring a mass flow rate. In addition, a thermal type flow sensor of the substrate type is known recently. This type flow sensor has temperature sensitive resistive elements such as a heat resistive element and a temperature compensation resistive element, which are micro-processed on a semiconductor substrate made of silicon or the like through a micromachining technique. Attention is paid to the thermal type flow sensor of the silicon substrate type because of the small size, the low power consumption, and the low costs.
The thermal type flow sensor of the silicon substrate type is disclosed in, for example, JP-A 2004-012358 and JP-A 2004-340936.
In the disclosure of the publications, all of the resistors in the bridge circuit are formed of resistive elements of the same material on the same substrate. As a result, the resistive elements that constitute the bridge circuit have the same characteristics. As those resistive elements may vary with time at the same rate if the conditions are same, the total balance of the bridge circuit may not vary. Therefore, it is possible to maintain a precision in the measurement for a long period of time.
However, in the bridge circuit of the thermal type flow sensor, there are various resistive elements with different heat conditions respectively, for example, one of which is a heat resistive element that heats at a high temperature, and the other is a resistive element (temperature sensitive resistive element) that is used substantially at a room temperature. As a generally trend, the heat resistive element become thermally deteriorated as the element is used, and is liable to be deteriorated as compared with the remaining resistive element. Accordingly, the characteristics of the sensor may be varied with time. The deterioration of the heat resistive element causes the bridge balance to be varied. The above phenomenon leads to an error in the temperature difference control of the resistive element and the deterioration of a precision in the flow rate measurement.
A sensor disclosed in JP-A 2004-340936 is configured to sense the temperature of the heat resistive element indirectly by the aid of the temperature sensitive resistive element that is disposed in the vicinity of the heat resistive element, so that the temperature of the heat resistive element is controlled on the basis of the indirectly sensed temperature value. In this case, because the temperature sensitive resistive element for the temperature sensing element is lower in the temperature than the heat resistive element, it is possible to reduce the thermal deterioration of the temperature sensitive resistive element. However, in the above configuration, because a thermal transmission delay from the heat resistive element to the temperature sensitive resistive element occurs, a response is delayed as much. Also, because a reference power supply is used for a driver circuit, the sensor is affected by a fluctuation of the reference power supply.
Also, the characteristic variation of the sensor is caused by a stress of the mounted member. Most of the thermal type flow sensors have a sensor element fixed with an adhesive. Also, a sealant such as an epoxy resin is used to protect wire bonding for taking out an electric signal from the sensor element.
The above adhesive and sealant expand and contract due to a change in the surrounding temperature, and give a stress to the sensor element. When the stress occurs in the sensor element, a stress is exerted on the resistive elements being formed on the sensor element, and the resistance value of the resistive varies. Also, the degree of the generated stress varies with time. In particular, in the case of the resistive element being formed of a semiconductor such as silicon, the piezoelectric resistance coefficient is large, and the resistance variation rate is large. The above resistance variation causes the deterioration of the measurement precision.